Electric Heating Assisted Passive and Active Regeneration for Efficient Emission Controls of Diesel Engines

ABSTRACT

A method of heating an exhaust gas in an exhaust aftertreatment system including a Diesel Oxidation Catalyst (DOC) and a diesel particulate filter (DPF) is provided. The method includes heating the exhaust gas to a predetermined temperature to increase NO 2  generation in the DOC when the DPF is not actively regenerated, and heating the exhaust gas to reduce an exhaust temperature gradient when the DPF is actively regenerated. The exhaust temperature gradient is reduced by providing more heat proximate a wall of an exhaust conduit and less heat proximate a center of the exhaust conduit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/601,923, entitled “Electric Heating AssistedPassive and Active Regeneration for Efficient Emission Controls ofDiesel Engines,” filed Feb. 22, 2012, the content of which isincorporated herein by reference in its entirety.

FIELD

The present disclosure relates to exhaust aftertreatment systems fordiesel engines, and more particularly to electric heating and control toprovide assisted heating in the exhaust aftertreatment systems.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure and may notconstitute prior art.

Diesel engines have been used in a variety of applications such aslocomotives, marines and engine-generators. The U.S. EnvironmentalProtection Agency (EPA) and the California Air Resources Board (CARB),as well as other regulatory agencies around the world, impose strictlimitations on the contents of emissions from diesel engines, such asparticulate matter (PM), hydrocarbon (HC) and NOx. Accordingly, exhaustaftertreatment systems have been employed and generally include a DieselOxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), and an SCR(Selective Catalytic Reduction of NOx) to treat the exhaust gas and tocontrol emissions to atmosphere or the outside environment.

Various chemical reactions occur in the DOC and SCR to convert harmfulnitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbon(HC) into N₂, CO₂ and water. The DPF is designed to remove dieselparticulate matter (PM) from the exhaust gas. Normally these chemicalreactions would take place at high temperatures. With the use ofcatalysts, the chemical reactions can occur at much lower temperatures.Sufficient energy in the form of heat, however, must still be suppliedto the catalysts to expedite the chemical reactions. Therefore,performance of the exhaust aftertreatment system is highly dependent onthe temperature of the exhaust gas, which carries the desired energy andheat to the catalysts. The normal temperature of the exhaust gas,however, does not always meet requirements for the desired chemicalreactions. When the normal exhaust temperature is lower than the targettemperature, the exhaust aftertreatment system cannot effectively treatthe exhaust gas, resulting in higher emissions to the outsideenvironment.

One method of increasing the exhaust gas temperature is throughinjecting hydrocarbon upstream from a DOC either in the exhaust pipe orinside the cylinder during the exhaust stroke. This method increasesfuel consumption and also changes composition of the exhaust gas. Forexample, when fuel injection is injected in the exhaust, NO₂ generationin the DOC is significantly reduced. NO₂ is an effective reagent forpassive regeneration of DPF at much lower temperature range. Therefore,the reduced NO₂ generation adversely affects the passive regeneration ofthe DPF.

SUMMARY

In one form, a heater control module for controlling an electric heaterin an exhaust aftertreatment system is provided. The exhaustaftertreatment system includes a Diesel Oxidation Catalyst (DOC) and aDiesel Particulate Filter (DPF). The heater control module includes aheating mode determination module and a heater operating module. Theheating mode determination module selects a desired heating mode from aplurality of heating modes based on an engine load and a status of theDPF. The heater operating module operates the electric heater based onthe desired heating mode.

In another form, a method of heating an exhaust gas in an exhaustaftertreatment system including a Diesel Oxidation Catalyst (DOC) and aDiesel Particulate Filter (DPF) is provided. The method includes heatingthe exhaust gas to a predetermined temperature to increase NO₂generation in the DOC when the DPF is not actively regenerated and whenthe engine load is low, and heating the exhaust gas to reduce an exhausttemperature gradient when the DPF is actively regenerated. The increasedNO₂ generation facilitates passive regeneration of the DPF. The exhausttemperature gradient is reduced by providing more heat proximate a wallof an exhaust conduit and less heat proximate a center of the exhaustconduit. The efficiency of the active regeneration of the DPF can berequired heat for active regeneration of DPF can be increased due tomore efficient burning of PM at the peripheral channels of the DPF.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, incorporated in and forming a part of thespecification, illustrate several aspects of the present invention and,together with the description, serve to explain the principles of theinvention. The components in the figures are not necessarily to scale.In the drawings:

FIG. 1 is a schematic view of an engine system including a heatingmodule constructed in accordance with the teachings of the presentdisclosure;

FIG. 2 is a schematic view of a heating module constructed in accordancewith the teachings of the present disclosure;

FIG. 3 is a graph showing relationship between concentration of NO₂ andcatalyst temperature;

FIG. 4 is a schematic view of an electric heater;

FIG. 5 is a graph showing a heating strategy for operating the electricheater;

FIG. 6 is a table showing the properties of the exhaust gas at differentengine loads; and

FIG. 7 is a schematic view of another form of an engine system includinga heating module constructed in accordance with the teachings of thepresent disclosure.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is in no wayintended to limit the present invention, its application, or uses. Itshould also be understood that steps within a method may be executed indifferent order without altering the principles of the invention.

Referring to FIG. 1, an engine system 10 generally includes a dieselengine 12, a generator 14, a turbocharger 16, and an exhaustaftertreatment system 18. The exhaust aftertreatment system 18 isdisposed downstream from a turbocharger 16 for treating exhaust gasesfrom the diesel engine 12 before the exhaust gases are released toatmosphere. The exhaust aftertreatment system 18 includes a heatingmodule 20, a DOC 22, DPF 24, and an SCR 26. The heating module 20includes an electric heater 28 disposed upstream from the DOC 22, and aheater control module 30 for controlling operation of the electricheater 28. The exhaust aftertreament system 18 includes an upstreamexhaust conduit 32 that receives the electric heater 28 therein, anintermediate exhaust conduit 34 in which the DOC 22 and DPF 24 arereceived, and a downstream exhaust conduit 36 in which the SCR isdisposed.

The DOC 22 is disposed downstream from the electric heater 28 and servesas a catalyst to oxide carbon monoxide and any unburnt hydrocarbons inthe exhaust gas. In addition, The DOC 22 converts harmful nitric oxide(NO) into nitrogen dioxide (NO₂). The DPF 24 is disposed downstream fromthe DOC 22 to remove diesel particulate matter (PM) or soot from theexhaust gas. The SCR 26 is disposed downstream from the DPF 24 and, withthe aid of a catalyst, converts nitrogen oxides (NOx) into nitrogen (N₂)and water. A urea water solution injector 27 is disposed downstream fromthe DPF 24 and upstream from the SCR 26 for injecting urea watersolution into the stream of the exhaust gas. When urea water solution isused as the reductant in the SCR 18, NOx is reduced into N₂, H₂O and CO₂in the following reaction:

4NO+2(NH₂)₂CO+O₂→4N₂+4H₂O+2CO₂

The electric heater 28 provides assisted heating of the exhaust gasflowing in the exhaust conduits 32, 34, 36. The generator 14 isconnected to the diesel engine 12 to drive the diesel engine 12 duringengine startup as an option and to supply electricity to the electricheater 34 during normal engine operation. The heater control module 30strategically controls the electric heater 28 in different heating modesto facilitate both active and passive regeneration of the DPF 24.

Regeneration is the process of burning and removing the accumulatedparticulates matters from the DPF 24. Regeneration can occur passivelyor actively. Passive regeneration can occur in normal engine operationwhen the temperature of the exhaust gas is sufficiently high. Activeregeneration can occur based on a monitored DPF condition or based on apredetermined timing schedule by introducing very high heat to theexhaust aftertreatment system 10. Active regeneration can be achieved byproper engine control management to increase the exhaust temperaturethrough late fuel injection or injection during the expansion stroke.Active regeneration can also be achieved through assisted heating by anelectric heater. Active regeneration requires much more heat thanpassive regeneration and thus subjects the ceramic structure of the DPF24 to the risk of cracking and decreases catalytic coating life time.

Referring to FIG. 2, the heater control module 30 strategically controlsoperation of the electric heater 28 based on an engine load and a statusof the DPF 24 to provide assisted heating in both active and passiveregeneration of the DPF. The heater control module 24 may be a part ofan engine control unit (ECU) (not shown) or external to the ECU. The ECUcontrols operation of the diesel engine 12, a fuel injection system (notshown), among others, and acquires and stores various parametersrelating to engine operating conditions, including but not limited to,exhaust temperature, diesel engine load, flow conditions (air flow andair pressure etc.) The heater control module 30 receives inputs from theECU to make the proper determination how to operate the electric heater28. The control module could also receive information from stand aloneafter treatment control systems.

The heater control module 30 includes a heating mode determinationmodule 32 and a heater operating module 33 including a passiveregeneration heating module 34 and an active regeneration heating module36. The electric heater 22 can be operated in two operating modes:passive regeneration heating mode and active regeneration heating mode.The heating mode determination module 32 determines a desired heatingmode based on an engine load and the status of the DPF 24. When the DPF24 is actively regenerated, the desired heating mode is the activeregeneration heating mode. When the DPF 24 is not actively regeneratedand the engine load is low, for example, at 10%, the desired heatingmode is the passive regeneration heating mode. The heating modedetermination module 32 may include a heating strategy that specifiesthe correlation among the heating modes, duration, engine loads and thedesired exhaust temperature rise. The heating mode determination module32 also determines when the electric heater 28 should be turned on oroff during normal engine operation. In response to the determination ofthe heating mode determination module 32, the heater operating module 33operates the electric heater 28 accordingly.

In the passive regeneration heating mode, the electric heater 28 iscontrolled to heat the exhaust gas to a predetermined temperature whichallows for optimum NO₂ generation in the DOC 22. NO₂ is an effectivereactant for passive regeneration of DPF 24. Increasing NO₂ generationcan facilitate passive regeneration of DPF 24. In the active heatingmode, the electric heater 28 is controlled to heat the exhaust gasdifferently to reduce exhaust temperature gradient across the exhaustconduits. When the temperature gradient is reduced, the activeregeneration can be accomplished more efficiently.

When the heating mode determination module 32 determines that thepassive heating mode is desired, the passive regeneration heating module34 then controls the electric heater 28 to heat the exhaust gas to apredetermined temperature. The passive regeneration heating module 34calculates and determines the desired temperature rise based on anexhaust temperature and the predetermined temperature. The exhausttemperature may be obtained from the input from the ECU, temperaturesensors. The predetermined temperature depends on the properties of thecatalysts in the DOC 14 and is set to allow for optimum NO₂ generation.

Referring to FIG. 3, the NO₂ concentration at the outlet of the DOC 14is dependent on the temperature of the exhaust gas. For a BASF DOCcatalyst, the NO₂ concentration is relatively high when the catalysttemperature is in the range of 300 to 460° C., particularly in the rangefrom 320 to 380° C. Therefore, the predetermined temperature is set tobe in the range of 300 to 460° C., and preferably in the range from 320to 380° C. When the electric heater 28 heats exhaust gas to thepredetermined temperature, an optimum amount of NO₂ is generated tofacilitate passive regeneration of the DPF 24. With the extensivepassive regeneration of DPF, the particulate matter is accumulated onthe DPF at a lower rate, thereby reducing the frequency for activeregeneration. As a result, the likelihood of DPF ceramic cracking anddegradation of the catalysts due to high heat associated with activeregeneration (generally in the range of 500 to 650° C.) are reduced.

Referring back to FIG. 2, when the DPF 24 is actively regenerated, thedesired heating mode is the active regeneration heating mode. The activeregeneration heating module 36 controls the electric heater 28 toprovide differential heating to the exhaust gas. The electrical heater22 generates more heat along the periphery of the electric heater andless heat at the center of the exhaust conduit.

The exhaust conduit generally has a relatively higher temperature alongthe central axis of the conduit and a relatively lower temperatureproximate the conduit wall. To ensure effective active regenerationacross the DPF 24, the exhaust gas proximate the exhaust conduit wallalso needs to be heated to the desired active regeneration temperature.Due to the temperature gradient across the cross section of the exhaustconduit, the exhaust gas proximate the center of the exhaust conduit isunnecessarily overheated, subjecting the center portion of the DPF 24 tohigher heat and higher risk of cracks. By operating the electric heater28 to reduce the temperature gradient, less heat is required to heat theexhaust gas to the desired active regeneration temperature. Therefore,the likelihood of overheating at the center of the DPF and theaccompanying problems is reduced.

Referring to FIG. 4, an exemplary embodiment of the electric heater 28is shown to have a low watt density zone 40 proximate the center and ahigh watt density zone 42 along the periphery of the electric heater 28.The electric heater 28 can provide differential heating across theexhaust conduit.

The electric heater 28 is powered by the generator 32. The generator 32drives the diesel engine 30 during engine startup. After the dieselengine 30 starts to operate on its own, the generator 30 is driven bythe diesel engine 30 to generate electricity to power other electronicsor electrical devices. The heating strategy allows for use of availableelectricity generating capacity when it is not needed to power the otherelectrical and electronic systems during low engine load operation.

Referring to FIG. 5, the heating mode determination module 32 includes aheating strategy which specifies the correlations among the heatingmodes, the exhaust temperature rise, the engine loads. As shown in theexemplary diagram, when the engine load is low and the DPF backpressureis in the range of medium to high, the target exhaust temperature rise(delta) would be low and the electric heater 28 is operated in thepassive regeneration mode. For example, the electric heater 22 is in thepassive regeneration heating mode when the diesel engine 30 is operatingnear low load conditions such as 10% load. The electric heater 22demands less electric power from the generator 32 because the desiredtemperature rise (delta) is less than that for active regeneration andbecause less exhaust gas is generated from the diesel engine 30 due tothe low engine load.

As the engine load continues to increase, for example, from 10% to 25%,to 50%, to 75%, the electric heater 22 is turned off. Activeregeneration of DPF may be initiated when the engine load is low oraccording to a predetermined schedule to benefit from heating lowerexhaust mass flow. When the DPF is actively regenerated, for example, atan engine load of 25%, the electric heater is turned on and operated inthe active regeneration heating mode to provide differential heating.When the active regeneration is completed and the engine load starts toincrease, the electric heater 28 is turned off.

Referring to FIG. 6, the table illustrates the exhaust contents fordifferent load conditions. As shown, when the diesel engine is operatedunder the 10% load condition, the exhaust gas exhibits the lowestexhaust flow (1925 cfm) and the highest available specific NOx (6.8g/bhp-hr) among the 5 load conditions for a gen-set type of large dieselengine. For example, had the exhaust temperature been raised from 235 C(455 F) to a temperature that is within the DOC's NO2 generation sweettemperature window of 320 to 380° C., the DOC downstream of the heaterwill generate maximum amount of NO₂ due to higher available NOx underthis load engine condition. NO₂ passively oxidizes the particulatematter loaded DPF downstream of the DOC at its maximum rate.Additionally, the delta T rise is only 85° C. which will minimize energyconsumption in comparison with an active regeneration which will have adelta T as high as 350° C.

For the 10% load condition on this Gen-set with a flow of 81.6 kg/min,it will require 121 KW energy input to heat the exhaust and have a deltaT rise of 85° C. It will need 450 KW to heat the exhaust up to 550° C.at 25% load condition with a flow of 137.3 kg/min.

For the notch 1 condition on a GE locomotive engine with a flow of 54.8kg/min, it will require 73 KW energy input to heat the exhaust and havea temperature rise (delta) of 76° C. up to 355° C. It will need 315 KWto heat the exhaust up to 607° C. at the same notch 1 condition.

With the extensive passive regeneration, the accumulation of the sootand PMs on the DPF 16, as well as the backpressure of the DPF, arereduced. As a result, the active regeneration periods and frequenciescan be significantly reduced, thereby enhancing durability of theexpensive DPF. The electric heating strategy of the present disclosuremay replace the fuel-injection-based active regeneration.

Referring to FIG. 6, the heating module 20 of the present disclosureapplies to all diesel engines which can generate electricity while inoperation, preferably to those non-EGR diesel engines having highengine-out NOx at lower duty cycles. As shown, the heating module 20 canbe applied to a catalyzed DPF only exhaust system, as well as an exhaustaftertreatment system 50 that includes DOC 52 and DPF 54 without SCR.

The heating module 20 of the present disclosure has at least thefollowing benefits:

1. Utilizing available electricity generating capacity when it is notneeded for other operations on a diesel-generator or a marine engine ora locomotive at low load to assist in passively regenerating the DPF aspart of the engine's emission control system.

2. Reducing the frequency of diesel fuel injection based activeregeneration and hence enhancing fuel economy of the engine operation.

3. Reducing DPF operational soot loading through heating assistedpassive regeneration to minimize overall operational backpressure.

4. Reducing risks of DPF cracking caused by soot overloaded runawayregenerations through heating assisted passive regeneration.

5. Improving exhaust aftertreatment system's performance throughdelivering more uniform exhaust temperatures across the system's inletface.

Additionally, the present disclosure may include methods of heatingportions of the gas flow in a more indirect matter. For example, thesystem could sense cooler portions within the gas flow cross section andprovide heat where needed to provide a more even temperaturedistribution and compensate for heat losses. In addition, for systemsthat require more electricity than is available to regenerate the entiregas stream cross-section, the system may regenerate in certain sectionsor zones at different times. These alternate forms of the presentdisclosure would also have a corresponding heater type that supportszone heating across the cross-section of gas flow, such as, by way ofexample, layered heaters or modular heat trace heaters such as thosedisclosed in pending U.S. application Ser. No. 11/238,747 titled“Modular Layered Heater System” and in U.S. Pat. No. 7,626,146 titled“Modular Heater Systems,” both of which are commonly assigned with thepresent application and the contents of which are incorporated byreference herein in their entirety.

The broad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited sincemodifications will become apparent from the following claims.

What is claimed is:
 1. A heater control module for controlling anelectric heater in an exhaust aftertreatment system including a dieseloxidation catalyst (DOC) and a diesel particulate filter (DPF), theheater control module comprising: a heating mode determination modulethat selects a desired heating mode from a plurality of heating modebased on an engine load and a status of the DPF; and a heater operatingmodule that operates the electric heater based on the desired heatingmode.
 2. The heater control module according to claim 1, wherein theplurality of operating modes include a passive regeneration heating modeand an active regeneration heating mode.
 3. The heater control moduleaccording to claim 2, wherein the electric heater is operated in theactive regeneration heating mode to provide differential heating whenthe DPF is actively regenerated.
 4. The heater control module accordingto claim 3, wherein the electric heater is operated in the activeregeneration heating mode to reduce a temperature gradient across anexhaust conduit.
 5. The heater control module according to claim 1,wherein the electric heater is operated in the passive regenerationheating mode to heat an exhaust gas to a predetermined temperature toincrease NO₂ generation when the DPF is not actively regenerated.
 6. Theheater control module according to claim 5, wherein the electric heateris operated in the passive regeneration heating mode when the engineload is low.
 7. The heater control module according to claim 6, whereinthe engine low is approximately 10%.
 8. The heater control moduleaccording to claim 5, wherein the predetermined temperature depends onproperties of catalysts in the DOC.
 9. The heater control moduleaccording to claim 8, wherein the predetermined temperature is in arange from 300 to 460° C.
 10. The heater control module according toclaim 8, wherein the predetermined temperature is in a range from 320 to380° C.
 11. A method of heating an exhaust gas in an exhaustaftertreatment system including a Diesel Oxidation Catalyst (DOC) and adiesel particulate filter (DPF), the method comprising: heating anexhaust gas to a predetermined temperature when the DPF is not activelyregenerated; and heating the exhaust gas to reduce an exhausttemperature gradient when the DPF is actively regenerated.
 12. Themethod according to claim 11, wherein the predetermined temperaturedepend on properties of the DOC.
 13. The method according to claim 11,further comprising increasing NO₂ concentration at an outlet of the DOCwhen the DPF is not actively regenerated.
 14. The method according toclaim 11, wherein the predetermined temperature is in a range from 300to 460° C.
 15. The method according to claim 11, wherein thepredetermined temperature is in a range from 320 to 380° C.
 16. Themethod according to claim 11, further comprising providing more heatproximate a wall of an exhaust conduit and less heat proximate a centerof the exhaust conduit to reduce the exhaust temperature gradient.